1. Field of the Invention
The present invention generally relates to position detecting systems and active RFID (Radio Frequency Identification) terminals. More particularly, the present invention relates to a position detecting system for position detecting by using an active RFID terminal and the RFID terminal.
2. Description of the Related Art
FIG. 1 is a structural view of an example of a related art position detecting system. As shown in FIG. 1, plural receivers 12a through 12j are fixed within a position detecting area 10. In addition, an active RFID terminal 14 moves in the position detecting area 10.
The active RFID terminal 14 sends a identification information signal by itself within a designate time interval. A reaching area (receivable area) of the identification information signal is shown by a circle 15. Because of this, the receivers 12a, 12b, 12c, and 12d situated in the circle 15 receive the identification information signal from the active RFID terminal 14.
When each of the receivers 12a through 12j receives the identification information signal, ID information demodulated from the identification information signal is sent to a server 16 via, for example, a LAN (Local Area Network).
When the server 16 receives the notification of the ID information from the receivers 12a, 12b, 12c, and 12d, which have received the identification information signal from the active RFID terminal 14, the server 16 calculates center of gravity 17 of positions of the receivers 12a, 12b, 12c, and 12d so as to estimate the center of gravity 17 as a present position of the active RFID terminal 14.
FIG. 2 is a block diagram of the RFID terminal 14 shown in FIG. 1. As show in FIG. 2, the ID information for identifying the RFID terminal is stored in an ID-ROM part 20. A transmission control part 21 codes the ID information read from the ID-ROM part 20 and supplies coded ID information to the RF part 22 with a designated time interval. The RF part 22 modulates the coded ID information and transmits it as an identification information signal of a specific frequency from an antenna 23.
FIG. 3 is a structural view of another example of the related art position detecting system. In FIG. 3, parts that are the same as the parts shown in FIG. 1 are given the same reference numerals, and explanation thereof is omitted.
As shown in FIG. 3, when a server 18 receives the ID information from the receivers 12a, 12b, 12c, and 12d receiving the identification information signal from the active RFID terminal 14, the server 18 weights each of positions of the receivers 12a, 12b, 12c, and 12d by using larger weights as the number of identification information signals received by the receivers 12a, 12b, 12c, and 12d is larger. Then the server 18 calculates the center of gravity 19 so as to estimate the center of gravity 19 as a present position of the active RFID terminal 14.
Here, the number of receptions of the identification information signals by the receiver 12c near the active RFID terminal 14 is large. The number of receiving the identification information signal by the receiver 12a far from the active RFID terminal 14 is small. The present position of the active RFID terminal 14 is estimated by being biased to a side of the receiver 12c where the number of received signal is large by weighting the number of received signals so that the position estimation precision of the active RFID terminal 14 is improved.
Patent Publication 1 mentioned below describes that an electric wave transmitted from the RFID is received by plural receivers so that the position of the RFID is specified by positions of the plural receivers.
Patent Publication 2 mentioned below describes that an electric wave transmitted from the portable terminal is received by plural receivers so that the position of the portable terminal is specified based on each of received levels at which plural receivers receive.
Patent Publication 3 mentioned below describes that an electric wave transmitted from the RFID is received by plural receivers and a position corresponding to the receiver closest to the RFID is specified as the position of the RFID based on the received radio strength of plural receivers.
Patent Publication 4 mentioned below describes that an electric wave transmitted from the portable terminal is received by plural local control stations and the position of the portable terminal is specified by position of the local control station or received radio strength.    [Patent Document 1] Japanese Laid-Open Patent Application Publication No. 10-124781    [Patent Document 2] Japanese Laid-Open Patent Application Publication No. 8-307917    [Patent Document 3] Japanese Laid-Open Patent Application Publication No. 11-224400    [Patent Document 4] Japanese Laid-Open Patent Application Publication No. 11-169353
However, in the above-discussed related art cases, the position of the RFID is specified based on the position of the receiver which receives the identification information signal or by weighting the number of receptions of the identification information signal to the position of the receiver. Therefore, there is limitation of precision for specifying the position of the RFID. In addition, in an area where the electric wave reaches, depending on the difference of frequency of the electric wave or reflection, there may be a position where the electric wave is not easily received by the receiver. This may cause degradation of precision for specifying the position of the RFID.